Ultrafiltration devices employing semipermeable membranes have been employed to separate aqueous solutions containing long-chain or polymeric components, such as the concentration of whey solutions and the concentration of paint from aqueous electro-coating paint compositions. Such ultrafiltration devices provide an aqueous-permeate fraction, while the high molecular weight or larger size molecular components of the solution are retained by the membrance and removed from the feed zone in a concentrated form. Ultrafiltration devices typically operate at pressures ranging from about 20 to 250 psig as distinct from reverse osmosis devices which operate at substantially higher pressures and require a different or much tighter membrane structure.
Typical membranes employed in such devices would comprise filament-wound or braided, glass-fiber, resin-reinforced tubes with a cellulose-acetate semipermeable membrane inserted or cast onto the interior surface of the tube support. Such supported membrane tubes rapidly diminish in flux rate with use when employed in an attempt to concentrate by ultrafiltration techniques, polyvinyl-chloride latex emulsions. It has been found, for example, that such regular supported membrane tubes lose their flux value irreversibly during the attempted concentration by ultrafiltration of polyvinyl-chloride latex. The reason for this loss of flux has not been conclusively determined, but it is believed to be associated with the fouling of the membrane by components of the polymer of the latex to be separated or concentrated. In the use of ultrafiltration devices for the separation of whey solutions in the dairy industry, the flux value of the membrane can often be somewhat restored by periodically cleaning the surface of the membrane with an aqueous detergent composition, which cleaning extends the useful life of the membrane, but typically never restores the flux to its prior or virgin value.